The reactor automatically shut down at 8:59 p.m. The plant responded safely and without incident.

The station's Unit 2 remains under the unusual event declaration because it is using backup power from two diesel generators until offsite power is restored, expected to occur sometime this weekend. Under NRC regulations, an unusual event is declared when the number of power redundancies to a plant is reduced. The loss of power is believed to be due to a problem with an onsite transformer.

As part of the plant's normal shutdown operations from loss of offsite power, steam is released and quickly evaporates, making a loud noise. There is no health or safety impact to workers or to the public from the release, but the sound can be startling to someone close by. Plant personnel notified local authorities and plant neighbors of the release Thursday evening.

Braidwood's Unit 1 reactor continues to operate at full power. The station is working to determine the cause of the power interruption.Exelon Nuclear personnel notified all appropriate federal, state and local government officials of the unusual event, the lowest of four emergency classifications as defined by the Nuclear Regulatory Commission.

Braidwood Generating Station is approximately 60 miles southwest of Chicago.